The systemic rheumatic disease are characterized by the presence of autoantibodies to nuclear antigens. Immune complexes containing nuclear antigens have been implicated in the pathogenesis of systemic lupus erythematosus (SLE). Since nuclear antigens are constantly release by cell death, mechanisms must exist for their removal from the circulation. It has recently been proposed that C-reactive protein (CRP) may act to clear nuclear material. CRP is an acute phase protein synthesized by the liver and normally maintained at <5microgram/ml in the circulation. In response to inflammation, infection or injury, CRP is synthesized rapidly producing circulating levels up to 500 microgram/ml. CRP was originally identified by its binding to pneumococcal C-polysaccharide, a reaction which is calcium-dependent and specific for phosphocholine (PC). Studies using PC- containing ligands have documented the ability of CRP to activate complement and to function as an opsonin in vivo and in vitro. CRP binds with high avidity to chromatin and to nucleosome core particles, the basic subunit of chromatin. CRP binding to nuclei of cells in synovial biopsies from rheumatoid arthritis patients and to nuclei of tissue culture cells has also been described. The proposed studies will investigate the role of CRP in the clearance of nuclear material and prevention of autoimmune responses. Initial studies have established the binding of CRP to histones H1, H2A and H2B and to the 70K protein of the U1 ribonucleoprotein complex. These reactions take place through the calcium-dependent PC binding site on CRP and suggest a shared epitope between several nuclear antigens. Immunoprecipitation and Western blotting of antigens extracted from cells in culture or rabbit thymus extract will be used to test for additional reactivities of CRP for nuclear proteins. Binding of CROP to fragments of histones and the 70K protein will be tested to determine the epitope(s) recognized by CRP. The effect of CRP on clearance of core particles from the circulation and on immunization with chromatin will be directly tested in a mouse model. CRP and complement have been reported to accelerate the degradation of chromatin into a soluble form. This solubilization reaction will be examined in more detail to establish the role of complement components, the composition of the soluble product and the possible contribution of nuclease to the process. There is evidence for a specific receptor for CRP on human and mouse mononuclear phagocytes which mediates ingestion of CRP-coated particles. This receptor is apparently present on human monocytes, polymorphonuclear leukocytes, and several monocyte cell lines, but has not been purified. U937 cells and human peripheral blood monocytes will be used to biochemically characterize the CRP receptor and to develop monoclonal antibodies to the receptor for use in further functional studies. These approaches will examine at several levels a CRP- dependent mechanism for the clearance of nuclear antigens. It is of interest to note that SLE patients generally have low CRP responses to the inflammation associated with their disease, suggesting an abnormality in this clearance mechanism.